1. Field of the Invention
The present invention relates to an objective lens driving apparatus for controlling operation to move an objective lens in the seeking direction, in the tracking direction, and in the focusing direction relative to a magneto-optical recording medium of a disk shape, and to an optical head provided with first and second bias magnetic field generating apparatuses for performing information recording by respective modulation methods different from each other in a small region at which a laser beam through the objective lens forms a beam spot, and specifically, to a magneto-optical disk apparatus provided with such an optical head.
2. Related Background Art
A magneto-optical disk apparatus of this type is required to be capable of recording and/or reproducing information in either of magneto-optical recording media in which information is recorded in the optical modulation method and those in which information is recorded in the magnetic field modulation method. In the optical modulation method, a constant magnetic field is applied by a bias magnetic field generating apparatus to a recording region on a magneto-optical recording medium and a laser beam modulated according to recording information is applied from an optical head to the recording region. The magneto-optical recording medium for the optical modulation method is for example a 3.5 inch magneto-optical disk, which is based on the first-generation ISO (as will hereinafter be referred to as ISO). In contrast, in the magnetic field modulation method, a constant laser beam is applied from an optical head to a recording region and a magnetic field modulated according to recording information is applied from a bias magnetic field generating apparatus to the recording region. In summary, the magneto-optical disk apparatus must secure the compatibility in use with such media.
Thus, the above magneto-optical disk apparatus employs such a structure that either of two types of bias magnetic field generating apparatus can be selectively used, as shown in FIG. 1, FIG. 2A, and FIG. 2B. One of the two bias magnetic field generating apparatuses is a fixed bias magnetic field generating apparatus as denoted by numeral 102, which applies a constant magnetic field to a magneto-optical recording medium for optical modulation recording based on the ISO to perform recording based on recording information from the optical head, while the other of the two bias magnetic field generating apparatuses is a bias magnetic field generating apparatus as denoted by numeral 101 for the magnetic field modulation recording method, which has a magnetic field generating portion (not shown) with less turns for modulating a magnetic field at a high speed. The bias magnetic field generating apparatus 101 is to be kept at a gap of several .mu.m to a disk surface of the magneto-optical recording medium in order to obtain a necessary magnetic field strength in a restricted small region, and is provided with a control mechanism (not shown) for that purpose.
A laser beam emitted from a light source (not shown) of a stationary optical system is reflected for example by a bending mirror and is then converged by an objective lens 103 to form a beam spot on a recording layer of the magneto-optical recording medium. The objective lens 103 is supported by a lens holder 103A, and the lens holder 103A is mounted on an objective lens driving apparatus 105 through an elastic support mechanism 103B such as a plate spring. The lens holder 103A is moved to effect focusing of the beam through the objective lens 103 on the recording medium, and the objective lens driving apparatus 105 is moved to effect the seeking operation and the tracking operation with the beam.
A constant magnetic field is applied to a small region including the beam spot and its neighboring by a magnetic field generating portion (magnetic head) arranged in the bias magnetic field generating apparatus 101 for magnetic field modulation recording. Information is written in the recording medium in correspondence with the polarity of the magnetic field. The bias magnetic field generating apparatus 101 is fixed onto a second driving mechanism 104. The objective lens driving apparatus 105 and the driving mechanism 104 are controlled independently to move in the radial direction of the disk.
Appropriate driving means (not shown) makes the fixed bias magnetic field generating apparatus 102 step back in the direction of the arrow a when the bias magnetic field generating apparatus 101 is used in the modulated magnetic field recording method (see FIG. 2A). Then, the bias magnetic field generating apparatus 101 becomes ready to move over the entire recording region on the magneto-optical disk (magneto-optical recording medium), and the fixed bias magnetic field generating apparatus 102 will never interfere with the bias magnetic field generating apparatus 101 and the driving mechanism thereof.
As described, the conventional example as shown in FIG. 1, FIG. 2A, and FIG. 2B is provided with the two types of bias magnetic field generating apparatuses, preparing for use with the magneto-optical disk for optical modulation recording based on the ISO and for use with the magneto-optical disk for magnetic field modulation recording. The fixed bias magnetic field generating apparatus 102 steps up over the bias magnetic field generating apparatus 101 in the conventional example while the bias magnetic field generating apparatus 101 is in operation for magnetic field modulation recording, which is disadvantageously against a desire to make the apparatus more compact, simpler, and thinner. In particular, complicated means or mechanisms are necessary for detecting a relative position between the objective lens driving apparatus and the respective bias magnetic field generating apparatuses and for positioning the apparatuses, which inevitably causes the structure of the apparatus to be complicated.
It is also necessary that the driving portion of the bias magnetic field generating apparatus 101 be withdrawn or moved in the radial direction of the recording medium in order to bring the fixed bias magnetic field generating apparatus 102 closer to the vicinity of the disk surface for optical modulation recording. A substantial length is required for the withdrawal of the driving portion and a mechanism for withdrawal or movement is necessary, which also makes the apparatus complicated.
Further, the bias magnetic field generating apparatus 101 for magnetic field modulation generates a relatively weak magnetic field as a result of priority on frequency properties. This increases a possibility of degradation of a C/N (carrier to noise) ratio for recording signals in magnetic field modulation recording because of influences from a floating magnetic field of a magnetic circuit for driving the objective lens of the optical head and from a residual field of the fixed bias magnetic field generating apparatus, as shown in FIG. 3. FIG. 3 shows the C/N ratio in the magnetic field modulation recording against the floating magnetic field (DC magnetic field at H.sub.1) during magnetic field modulation.